‘Hot Jupiters’: Explaining Spin-Orbit Misalignment

byPaul GilsteronSeptember 16, 2014

Bringing some order into the realm of ‘hot Jupiters’ is all to the good. How do these enormous worlds get so close to their star, having presumably formed much further out beyond the ‘snowline’ in their systems, and what effects do they have on the central star itself? And how do ‘hot Jupiter’ orbits evolve so as to create spin-orbit misalignments? A team at Cornell University led by astronomy professor Dong Lai, working with graduate students Natalia Storch and Kassandra Anderson, has produced a paper that tells us much about orbital alignments and ‘hot Jupiter’ formation.

It’s no surprise that large planets — and small ones, for that matter — can make their stars wobble. This is the basis for the Doppler method that so accurately measures the movement of a star as affected by the planets around it. But something else is going on in ‘hot Jupiter’ systems. In our own Solar System the rotational axis of the Sun is more or less aligned with the orbital axis of the planets. But some systems with ‘hot Jupiters’ have shown a misalignment between the orbital axis of the gas giants and the rotational axis of the host star.

Image: ‘Hot Jupiters,’ large, gaseous planets in inner orbits, can make their suns wobble after they wend their way through their solar systems. Credit: Dong Lai/Cornell University.

The Cornell team went to work on simulations of such systems, working with binary star systems separated by as much as hundreds of AU. Their work shows that gas giants can be influenced by partner binary stars that cause them to migrate closer to their star. At play here is the Lidov-Kozai mechanism in celestial mechanics, an effect first described by Soviet scientist Michael Lidov in 1961 and studied by the Japanese astronomer Yoshihide Kozai. The effect of perturbation by an outer object is an important factor in the orbits of planetary moons, trans-Neptunian objects and some extrasolar planets in multiple star systems.

Thus the mechanism for moving a gas giant into the inner system, as described in the paper:

In the ‘Kozai+tide’ scenario, a giant planet initially orbits its host star at a few AU and experiences secular gravitational perturbations from a distant companion (a star or planet). When the companion’s orbit is sufficiently inclined relative to the planetary orbit, the planet’s eccentricity undergoes excursions to large values, while the orbital axis precesses with varying inclination. At periastron, tidal dissipation in the planet reduces the orbital energy, leading to inward migration and circularization of the planet’s orbit.

As the planet approaches the star, interesting things continue to occur. From the paper:

It is a curious fact that the stellar spin axis in a wide binary (~ 100 AU apart) can exhibit such a rich, complex evolution. This is made possible by a tiny planet (~ 10-3 of the stellar mass) that serves as a link between the two stars: the planet is ‘forced’ by the distant companion into a close-in orbit, and it ‘forces’ the spin axis of its host star into wild precession and wandering.

Moreover, “…in the presence of tidal dissipation the memory of chaotic spin evolution can be preserved, leaving an imprint on the final spin-orbit misalignment angles.”

The approach of the ‘hot Jupiter’ to the host star can, in other words, disrupt the previous orientation of the star’s spin axis, causing it to wobble something like a spinning top. The paper speaks of ‘wild precession and wandering,’ a fact that Lai emphasizes, likening the chaotic variation of the precession to chaotic phenomenon such as weather systems. The spin-orbit misalignments we see in ‘hot Jupiter’ systems are thus the result of the evolution of changes to the stellar spin caused by the migration of the planet inward.

The paper goes on to mention that we see examples of chaotic spin-orbit resonances in our own Solar System. Saturn’s satellite Hyperion experiences what the paper calls ‘chaotic spin evolution’ because of resonances between its spin and orbital precession periods. Even the rotation axis of Mars undergoes chaotic variation due to much the same mechanism.

Interesting point about Mars. The obliquity, or tilt of a planet has a huge effect on its climate and habitability. The Earths obliquity only varies from 21.5-24.5 degrees over 41 k years or so so things remain pretty consistent ,although even this can lead to ice ages. There are many reasons for this not least the damping effect of a large nearby moon. One of the many causes of Mars’ inhabitability is its wild variations in obliquity . At present it is slightly more than Earths 23.5 degrees , but it can be over 90 degrees, leading to a tilt like Uranus. This appears to be because bizarrely it is in secular resonance for obliquity with every other planet in the solar system bar Earth. Why ? When it comes to planetary “resonance” , strange things start happening.

How much can a star’s rotation axis drift or precess in the absence of any nearby body that it orbits or is orbited by? Is it zero (or negligible) if stellar processes have no persistent asymmetrical internal features?

But aren’t many Hot Jupiters in single star systems? How do they get so close to their stars?

I think the paper covers this in the quote I give: “In the ‘Kozai+tide’ scenario, a giant planet initially orbits its host star at a few AU and experiences secular gravitational perturbations from a distant companion (a star or planet).”

I take this to mean that there are configurations where an outer planet can have this effect on an inner one.

Young stars rotate rapidly ,with some large O/B stars in particular reaching enormous speeds, getting close to that point where the centrifugal force outwards almost overcomes gravitational attraction inwards , threatening to pull the star apart. As the star is not solid this rotation varies according to latitude, being greatest at the equator causing the oblateness. The rotation throws off mass to which leads to a circumstellar ring of dust visible in the infrared as an emission line. As more mass is thrown out , and the star’s rotation leads to a large magnetic field too , this interacts with the stellar wind and the disk , acting as a brake but also leading to many of the irregularies that can be caused by gravitatonal interaction in the absence of other bodies. A further fomplicating factor , and something Kepler found ,is many stars have extremely close companion stars, so called eclipsing binaries that apart from complicating planetary searches by giving false positive results , also interact with the main star to cause gravitational interactions in the absence of any planets. Resonant interactions can lead to one or more of the bodies involved, both planet and close binaries , being thrown out of the system altogether. It has long been suspected that the early abnormal movements of Jupiter and Neptune in particular in our own system were caused by resonant interaction with another gas giant that was subsequently expelled .

The 55 Cancri system appears to have ALL of its five planets misaligned by slightly more than seventy degrees. Is this a result of 55 Cancri b, or, what I believe to be is the case, 55 Cancri e once being a Hot Jupiter, causing the star to presess wildly BEFORE its gas envelope completely evaporated, leaving the solid core completely exposed. The only problem with this scenario is that all the planets orbits would have to be made eccentric in a MATRIOSCHKA fashion for the system not to go COMPLETELY haywire. Then , how do the OTHER FOUR PLANETS wind up in their CURRENT orbits? Can 55 Cancri”s (or, more properly, 55 Cancri A’s) companoin star do this via the Kozai mechinism ALONE, or, are there OTHER forces at work here?

A planet may be causing the star it orbits to act much older than it actually is, according to new data from NASA’s Chandra X-ray Observatory. This discovery shows how a massive planet can affect the behavior of its parent star.

The star, WASP-18, and its planet, WASP-18b, are located about 330 light-years from Earth. WASP-18b has a mass about 10 times that of Jupiter and completes one orbit around its star in less than 23 hours, placing WASP-18b in the “hot Jupiter” category of exoplanets, or planets outside our solar system.

WASP-18b is the first known example of an orbiting planet that has apparently caused its star, which is roughly the mass of our sun, to display traits of an older star.

“WASP-18b is an extreme exoplanet,” said Ignazio Pillitteri of the Istituto Nazionale di Astrofisica (INAF)-Osservatorio Astronomico di Palermo in Italy, who led the study. “It is one of the most massive hot Jupiters known and one of the closest to its host star, and these characteristics lead to unexpected behavior. This planet is causing its host star to act old before its time.”

Pillitteri’s team determined – WASP-18 is between 500 million and 2 billion years old, based on theoretical models and other data. While this may sound old, it is considered young by astronomical standards. By comparison, our sun is about 5 billion years old and thought to be about halfway through its lifetime.

Younger stars tend to be more active, exhibiting stronger magnetic fields, larger flares, and more intense X-ray emission than their older counterparts. Magnetic activity, flaring, and X-ray emission are linked to the star’s rotation, which generally declines with age. However, when astronomers took a long look with Chandra at WASP-18 they didn’t detect any X-rays. Using established relations between the magnetic activity and X-ray emission of stars, as well as its actual age, researchers determined WASP-18 is about 100 times less active than it should be.

“We think the planet is aging the star by wreaking havoc on its innards,” said co-author Scott Wolk of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

The researchers argue that tidal forces created by the gravitational pull of the massive planet – similar to those the moon has on Earth’s tides, but on a much larger scale – may have disrupted the magnetic field of the star.

We know of many spectroscopic binary stars, extremely close together. I wonder if they started out so close together? And if binary stars can form that close together, why not binary Jupiters or binary brown dwarfs? I wonder what happens when they start migrating inwards?

@ashley baldwin – a large change in obliquity would have a huge impact on life on EARTH, but with a sample size of 1, it’s hard to say what the impact would be on life on another planet. You could easily say that a change in obliquity (which could take 10K-60K years) may stimulate evolution on the planet.

One other question – are there any hot Jupiter systems with more than one planet? I would expect the migrating hot Jupiter would disrupt all the inner planets, but may planets outside its initial orbit intact (and possibly with a different inclination).

Note that the low-eccentricity of the 55 Cancri planets means the Kozai mechanism is NOT operating: there are various things that can disrupt it, e.g. strong planet-planet interactions, tidal forces etc. (this is why Uranus can maintain a system of regular satellites despite the inclination exceeding the Kozai angle). In particular, dynamical studies show that the coupling between the 55 Cancri planets results in the system precessing as a rigid body under the influence of the companion star.

I have been puzzling about Tom Mazanec´s question and I think it is still valid and not (completely) answered, in fact a two-way question:
– Is a Hot Jupiter indicative of an (inclined) binary star (or other giant planet in outer orbit, seems less likely to me though)?
– Does an (inclined) binary star (always, mostly) lead to a Hot Jupiter and/or strongly eccentric planets?

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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